1. Field of the Invention
The present invention relates to an apparatus for and a method of automatically welding a flange to a guide thimble tube in the manufacturing process for the guide thimble tube, which is a major element of a nuclear fuel assembly.
2. Background of the Related Art
A nuclear fuel assembly is an assembly of fuel elements that can be handled in one entity when loaded into and unloaded from a nuclear reactor. For example, a nuclear fuel assembly used for a light-water reactor (called a fuel assembly) is composed of a plurality of fuel rods (fuel elements) forming a bundle of a rectangular shape, at both ends of which a perforated support plate is disposed to pass coolant, and at the intermediate of which a support grid is mounted for the fuel rods to be spaced apart from each other. The individual fuel rod is formed of a hollow zicaloy tube having a length of about 4 mm, inside of which sintered uranium pellets are stacked. Typically, the fuel bundle for a boiling water reactor is formed of about 60 fuel rods and one for a pressurized water reactor is formed of about 230 fuel rods.
In such fuel assembly, a skeletal structure to which fuel rod is loaded is called a “fuel assembly skeleton.” The fuel assembly skeleton is formed of an upper end fitting, a lower end fitting, a guide tube, an instrument tube, and so on. These components are connected to each other through a welding or other mechanical method. In the nuclear fuel assembly, the assembly skeleton functions to keep a gap between the individual fuel rods and hold them in place. The assembly skeleton forms a basic structure of the nuclear fuel assembly.
In the above assembly skeleton, the guide tube is an internal structure installed inside of a nuclear reactor for the purpose of guiding of smooth entry of the control rod. The starting, interrupting and output-controlling of a nuclear reactor are carried out by entering the control rod (a neutron absorber) into the reactor core. The guide tube has important roles in determining a proper location, forming a flow path to cool the fuel properly and so on, without damaging the integrity of the fuel assembly. In addition, the guide tube serves as a path for the poison rod, the nuclear source rod and so on, as well as the control rod.
Such a guide tube is fixed to under the upper end fitting and a control rod or the like is charged from the upper part of the reactor to enter the guide tube. The connection with the upper end fitting is carried out by a flange attached to one end of the guide. Thus, a flange must be attached to the guide tube.
The guide tube is structured such that its lower portion has a smaller inner diameter for various equipments charged from the upper part not to be damaged by impact when being dropped. This is to reduce the descending speed of the control rod. In addition, two flow holes are formed so that coolant water can be filled inside the guide tube.
The guide tube having the above-described structure had a sophisticated shape and thus it is very difficult to manufacture the guide tube in an automatic way. In addition, the guide tube plays a very important role in the nuclear reaction and thus must be manufactured with a great precision.
Therefore, since the welding of a flange or the like to the guide tube must be performed with a great precision, a manual manufacturing process has been carried out for the purpose of quality, which leads to a time consuming. However, considering the quantity of guide tubes to be manufactured, it must be considered a way of improving production efficiency.
A conventional welding method of connecting a flange to a guide tube is mostly performed manually. As previously mentioned, it is because the shape of the guide tube is not simple and thus an automatic welding is not easy. In particular, in order to weld a flange, as shown in FIGS. 1a and 1b, the guide tube and the flange are connected through an interference fit welding using manually-operated equipment.
Conventionally, as illustrated in FIG. 1a, a pressure-inserting step is formed in the flange 2 and thus as shown in FIG. 1b the pressure-inserting step is forcibly inserted into the tube 1. Then, a welding is carried out.
The above welding method requires a separate pressure-fitting process and forms an internal step during the pressure inserting, thereby necessitating a separate machining process. In addition, in case of manually working on a larger amount of guide tubes, there is a concern about diseases in the musculoskeletal systems, due to the tube length of about 4000 mm and weight of about 2.5 kg.
Furthermore, in terms of productivity and quality, the working time is extended and not consistent to degrade its production efficiency. In case of expediting the process in order to improve the productivity, the quality is degraded.